Advanced communications today may use the Orthogonal Frequency Division Multiplex (OFDM) modulation for efficient transmission of digital signals. These signals may include video, voice and/or data. OFDM is a commonly used implementation of Multi-Carrier Modulation (MCM).
The Orthogonal Frequency Division Multiplex (OFDM) is a modern advanced modulation method, that achieves better use of the frequency spectrum.
OFDM has been used in recent years in many applications where robustness against severe multipath and interference conditions is required, or a high system capacity, flexibility in providing variable bit rate services, scalability and a capability to perform well in Single Frequency Networks (SNF). OFDM forms the basis for various communication standards, including for example the Digital Terrestrial Television Broadcasting, wireless LANs and Wireless Local Loops.
OFDM requires an advanced signal processing. Thus, a block of information is divided among N frequency channels, so that a portion of the information is transmitted in each of the abovementioned channels or frequencies. Since each channel is orthogonal to the others, a better utilization of the frequency spectrum is achieved.
In OFDM, since each symbol is N times longer, the percent overlap between adjacent symbols decreases, hence the Inter-Symbol Interference ISI is lower. Still better spectrum utilization is achieved by QAM (Quadrature Amplitude Modulation) on each of the N carriers.
An IFFT (Inverse Fourier Transform) is performed on the modulated carriers, to form the signal in the time domain that corresponds to the above modulated carriers. The signal is transmitted as a frame that contains the block of information to be transmitted.
A possible problem in the above modulation scheme may be an error in the time synchronization between signals.
When there is a time synchronization error, the signals after FFT in the various subchannels are rotated with respect to each other. This effect creates interference within the subchannel
Another problem is a frequency error between the transmitted signal and the receiver. A frequency error generates a frequency shift that may change the location of symbols and/or may generate interference between symbols.
Because of channel imperfection, a time or phase delay may be generated between the various parts of the spectrum of the transmitted signal.
This distortion of the frequency spectrum of the transmitted signal may interfere with the signal reconstruction in the receiver.
The problem is further aggravated by multipath. Multipath may cause several replicas of a signal to be received, each possibly having a different time delay, amplitude and polarity.
These signals may result in interference between adjacent transmitted frames.
Prior art systems apparently are different.
Thus, Seki et al., U.S. Pat. No. 5,771,224. discloses an orthogonal frequency division multiplexing transmission system and transmitter and receiver therefor. It transmits an OFDM transmission frame, with null symbols and reference symbols being placed in the beginning portion of the frame and QPSK symbols are placed in an information symbol data region in the frame, with equal spacing in time and frequency.
The carrier amplitude and phase errors are corrected by a correction information producing section on the amplitude and phase variations of the received signal detected by the variation detector to produce corrected information.
Baum et al., U.S. Pat. No. 5,802,044, discloses a multicarrier reverse link timing synchronization system. A center station transmits a forward link signal, receives a reverse link signal, and determines a timing offset for signals received on a reverse link timing synchronization channel. A reverse link symbol timing synchronization can be used in a system having a plurality of transmitting overlap bandwidth subscriber units on an OFDM-like spectrally overlapping reverse channel. The modulation method may comprise M-ary Quadrature Phase Shift Keying(M-PSK), M-ary Quadrature Amplitude Modulation (QAM) or other digital modulation method.
Gudmundson et al., U.S. Pat. No. 5,790,516, discloses a method and system for pulse shaping for data transmission in an orthogonal frequency division multiplexed (OFDM) system.
Yamauchi, et al., U.S. Pat. No. 5,761,190, discloses an OFDM broadcast wave receiver. An OFDM (Orthogonal Frequency Division Multiplex) broadcast wave receiver for receiving an OFDM broadcast wave.
It automatically discriminates whether the received signal is of a wide band or a narrow band by determining if a carrier signal having a predetermined frequency is present among signals of a plurality of frequencies, acquired by OFDM demodulation of the reception signal by demodulation means.
It also controls the demodulating operation of the demodulation means in accordance with the discrimination result to thereby acquire a demodulated signal.
Schmidl, et al. U.S. Pat. No. 5,732,113, discloses a a method for timing and frequency synchronization of OFDM signals. It relates to a method and apparatus that achieves rapid timing synchronization, carrier frequency synchronization, and sampling rate synchronization of a receiver to an orthogonal frequency division multiplexed (OFDM) signal. The method uses two OFDM training symbols to obtain full synchronization in less than two data frames. A first OFDM training symbol has only even-numbered sub-carriers,
A second OFDM training symbol has even-numbered sub-carriers differentially modulated relative to those of the first OFDM training symbol by a predetermined sequence.
Synchronization is achieved by computing metrics which utilize the unique properties of these two OFDM training symbols. Timing synchronization is determined by computing a timing metric which recognizes the half-symbol symmetry of the first OFDM training symbol. Carrier frequency offset estimation is performed in using the timing metric as well as a carrier frequency offset metric which peaks at the correct value of carrier frequency offset. Sampling rate offset estimation is performed by evaluating the slope of the locus of points of phase rotation due to sampling rate offset as a function of sub-carrier frequency number.
Awater, et al. U.S. Pat. No. 5,862,182, discloses an OFDM digital communications system using complementary codes.
The encoding/transmission of information in an OFDM system is enhanced by using complementary codes. The complementary codes, more particularly, are converted into phase vectors and the resulting phase vectors are then used to modulate respective carrier signals. The modulated result is then transmitted to a receiver which decodes the received signals to recover the encoded information.
Isaksson, et al. U.S. Pat. No. 5,812,523, discloses a method and device for synchronization at OFDM-system.
A method of demultiplexing OFDM signals and a receiver for such signals.
The method is concerned with synchronization in an OFDM receiver. A signal is read into a synchronization unit, in the time domain, i.e., before Fourier transforming the signal by means of an FFT processor. In the synchronization unit, a frame clock is derived for triggering the start of the FFT process and for controlling the rate at which data is supplied to the FFT processor. For OFDM reception, it is vital that the FFT process commences at the right point in time. Once the frame clock has been recovered, a frequency error can be estimated by the synchronization unit. The frequency error is used to control an oscillator which generates a complex rotating vector which is, in turn, multiplied with the signal to compensate for frequency errors. The method can be used both with OFDM systems in which symbols are separated by guard spaces, and with OFDM systems in which symbols are pulse shaped.
Kim, U.S. Pat. No. 5,963,592, discloses an adaptive channel equalizer for use in digital communication system utilizing OFDM method. An adaptive channel equalizer for use in OFDM receiver is disclosed. The adaptive channel equalizer comprises a first complex multiplier for outputting a first in-phase complex multiplication signal and a first quadrature phase complex multiplication signal; a reference signal generator for generating a reference signal; an error calculator for outputting an in-phase error signal and a quadrature phase error signal; a delay unit for outputting an in-phase delay signal and a quadrature phase delay signal; a gain controller for outputting an in-phase gain control signal and a quadrature phase gain control signal;
a second complex multiplier for outputting a second in-phase complex multiplication signal and a second quadrature phase complex multiplication signal; an adder for outputting updated in-phase and quadrature phase coefficients; an address generator for generating a write address signal and a read address signal;
a storage unit for storing the updated in-phase and quadrature phase coefficients, and outputting the updated coefficients; an initial coefficients generator for generating an initial coefficients; a selecting signal generator for generating a selecting signal; and a multiplexing unit for selecting one of the initial coefficients and the updated coefficients according to the selecting signal.
Seki, et al., U.S. Pat. No. 5,694,389, discloses an OFDM transmission/reception system and transmitting/receiving apparatus. The apparatus improves the frequency acquisition range and the resistance to multipath interference. In a digital signal transmission system using OFDM, on the transmission side, some or all of a plurality of equidistant carrier positions are treated as reference carrier positions. The actual transmitted carriers are arranged in a predetermined pattern non-equidistant to the frequency carrier positions to form an OFDM symbol.
This OFDM symbol is periodically transmitted as frequency reference symbols. On the reception side, the carrier arrangement pattern of the frequency reference symbols is detected, a carrier frequency offset is detected from the detected pattern offset, and the carrier frequency is compensated based on the frequency offset.
Cimini, et al. U.S. Pat. No. 5,914,933, discloses a clustered OFDM communication system. A multicarrier communication system for wireless transmission of blocks of data having a plurality of digital data symbols in each block. The communication system includes a device for distributing the digital data symbols in each block over a plurality of clusters, each of the clusters receiving one or more digital data symbols. The digital data symbols are encoded in each of the cluster; and modulated in each cluster to produce a signal capable of being transmitted over the sub-channels associated with each cluster.
A transmitter thereafter transmits the modulated signal over the sub-channels. By distributing the modulated signal over a plurality of clusters, overall peak-to-average power (PAP) ratio is reduced during transmission and transmitter diversity is improved.
Williams, et al. U.S. Pat. No. 5,815,488, discloses a multiple user access method using OFDM. A communication method enables a plurality of remote locations to transmit data to a central location. The remote locations simultaneously share a channel and there is a high degree of immunity to channel impairments.
At each remote location, data to be transmitted is coded by translating each group of one or more bits of the data into a transform coefficient associated with a frequency in a particular subset of orthonormal baseband frequencies allocated to each remote location. The particular subset of orthonormal baseband frequencies allocated to each location is chosen from a set of orthonormal baseband frequencies. At each remote location, an electronic processor performs an inverse orthogonal transform (e.g., an inverse Fourier Transform) on the transform coefficients to obtain a block of time domain data. The time domain data is then modulated on a carrier for transmission to the central location.
Preferably, the time intervals for data transmission at the different remote locations are aligned with each other. In one embodiment of the invention, all of the baseband frequencies are allocated to a single particular remote location for one time slot. At the remote location, data is received from a plurality of remote locations. The data is demodulated to obtain baseband time domain data. The orthogonal transform is performed on this data to obtain transform coefficients. Each transform coefficient is associated with a baseband frequency. The central location keeps track of which baseband frequencies are allocated to which remote location for subsequent translation of each transform coefficient.
Isaksson, U.S. Pat. No. 5,726,973, discloses a method and arrangement for synchronization in OFDM modulation. A method and an arrangement for synchronization in OFDM modulation. Frequency errors of an IF clock and a sampling clock are controlled by estimating the deviation of the sampling clock and, respectively, the IF clock for two subcarriers with different frequencies.
According to the invention, the frequencies are chosen symmetrically around zero and the absolute phase errors are detected for both subcarriers.
Timing errors and phase errors are formed from the absolute phase errors in order to generate two control signals. The first control signal is formed from the deviation of the sampling clock and the timing error for controlling the sampling clock while the second control signal is formed from the deviation of the IF clock and the phase error for controlling the IF clock.
Wright, U.S. Pat. No. 5,838,734, discloses means for compensation for local oscillator errors in an OFDM receiver. A receiver for orthogonal frequency division multiplexed signals includes means for calculating the (discrete) Fourier Transform of the received signal, and means for calculating the phase error due to local oscillator errors.
McGibney, U.S. Pat. No. 5,889,759, discloses an OFDM timing and frequency recovery system. A synchronizing apparatus for a differential OFDM receiver that simultaneously adjust the radio frequency and sample clock frequency using a voltage controlled crystal oscillator to generate a common reference frequency. Timing errors are found by constellation rotation.
Subcarrier signals are weighted by using complex multiplication to find the phase differentials and then the timing errors. The reference oscillator is adjusted using the timing errors. Slow frequency drift may be compensated using an integral of the timing error. Frequency offset is found using the time required for the timing offset to drift from one value to another.
Background material on advanced modulation techniques and related communication topics may be found in the following articles:
Scott L. Miller and Robert j. O'Dea, “Peak Power and Bandwidth Efficient Linear Modulation”, IEEE transactions on communications, Vol. 46, No. 12, pp. 1639-1648, December 1998.
Kazuki Maeda and Kuniaki Utsumi, “Bit-Error of M-QAM Signal and its Analysis Model for Composite Distortions in AM/QAM Hybrid Transmission”, IEEE transactions on communications, Vol. 47, No. 8, pp. 1173-1180, August 1999.
Kazuki Maeda and Kuniaki Utsumi, “Performance of Reduced-Bandwidth 16QAM with Decision-Feedback Equalization”, IEEE transactions on communications, Vol. COM-35, No. 7, pp. 1173-1180, July 1987.
Background material on phase noise in advanced communication systems may be found in the following references:
Yossi Segal and Zion Hadad, “OFDMA access method for HIPERACESS”, HARNC1.doc, December 1999.
Naftali Chayat, “Updated Submission Template for TGa—Revision 2”, IEEE 802.11-98/156r2, March 1998.
Alcatel, Bosch, Ericsson, Lucent, Nokia, Siemens AG and Siemens ICN, “Proposal for the Adoption of the TDMA Access Scheme in HIPERACCESS”, HA16ERI1a.doc, December 1999.
Thierry Pollet, Mark Van Bladel and Marc Moeneclaey, “BER Sensitivity of OFDM Systems to Carrier Frequency Offset and Wiener Phase Noise”, IEEE transactions on communications, Vol. 43, No. 2/3/4, pp. 191-193, February/March/April 1995.
Luciano Tomba, “On the Effect of Wiener Phase Noise in OFDM Systems”, IEEE transactions on communications, Vol. 46, No. 5,pp. 580-583, May 1998.
Naftali Chayat, “TGa Comparison Matrix per 98/156r2”, IEEE 802.11-98/157r5, May 1998.
ETSI EP BRAN #16 Athens, Greece Nov. 29 —Dec. 3, 1999 HA16RNC1Annex.doc page 3 of 13 22-Nov-99